In this project, we are investigating how the brain stem controls the movements of the eyes and head when they cooperate to turn gaze toward an interesting object. Experiments from previous grant periods have shown that with the head restrained from moving, neurons in the reticular formation are connected together to produce the burst of spikes leading to a rapid movement of the eyes called a saccade. We now are asking whether this circuit in fact is controlling the overall gaze shift (i.e., eye plus head movement) when the head is free to move, or does it control just the eye movement component. We have made progress on this project on two fronts. First, brainstem neurons that exhibit a pause in firing for eye saccades with the head restrained also pause for eye saccades with the head free. Second, there are two types of brainstem neurons that discharge a burst of spikes during head-free gaze shifts. One is clearly related specifically to movements of the head and the other to movement of the eyes or gaze. Because the eye saccade and gaze shift have such similar time courses, it has been difficult to determine whether the burst is preferentially related to one or the other. However, the end of the burst is usually much better timed with the end of the eye movement than with the end of the gaze shift. Taken together, our preliminary recording data support our hypothesis that saccade-related neurons in the reticular formation encode eye but not gaze movement and that the eye and head movement components of a gaze shift are produced by separate neural circuits in the primate brain stem. Finally, we tested whether saccades are generated by a moving mound of neuronal activity in the superior colliculus, and found that they are not. FUNDING NIH grants RR00166 and EY00745. Soetedjo, R., Kaneko, C. R. S. and Fuchs, A. F. The timing of SC activity does not support the moving hill model. Soc. Neurosci. Abstr. 24 418, 1998.